1. Field of the Invention
The present invention relates generally to a method for fracturing a subterranean formation. More specifically, the invention is directed to a method and apparatus for placing multiple fractures in a horizontal or vertical openhole well.
2. Description of the Prior Art
In the recovery of oil and gas from subterranean formations it is common practice to fracture the hydrocarbon-bearing formation, providing flow channels for oil and gas. These flow channels facilitate movement of the hydrocarbons to the wellbore so they may be produced from the well. Without fracturing, many wells would cease to be economically viable.
In such fracturing operations, a fracturing fluid is hydraulically injected down a wellbore which penetrates the subterranean formation. The fluid is forced down the interior of the wellbore casing, through perforations, and into the formation strata by pressure. The formation strata or rock is forced to split or crack open, and a proppant is carried by the fluid into the crack and then deposited. The resulting fracture, with proppant in place to hold the crack open, provides improved flow of recoverable fluid, i.e., oil, gas, or water, into the wellbore.
Fracturing horizontal wells can significantly enhance well productivity, but the cost of multiple fracture completion according to the current industry practice is often unacceptably high. Therefore, operators often choose to complete wells, particularly horizontal wells, as open hole and in some cases, use slotted or preperforated liner or wire wrap screen to maintain hole integrity or provide solids exclusion.
One method currently used for multiple fracture completion is placing the fractures in stages (i.e., one fracture at a time at a wellbore location). Fracturing in stages has the advantage of precise fracture locations and design control, but is relatively expensive. A particular zone or interval is isolated using methods common in the industry, such as using retrievable or drillable bridge plugs with packers, sand or gravel, and a fluid. Well completion consists of setting a bridge plug below each target interval, perforating the target interval, pumping the fracture treatment, and cleaning out any sand remaining in the well bore to prepare for the same process for the next interval. This process repeats until all the target intervals are fractured. The bridge plugs then have to be retrieved or drilled out and well bore cleaned out to proceed with installation of production tubing. In some applications, sand plugs are set in the well bore for fracture isolation in lieu of bridge plugs. This method requires multiple trips into the well during the fracture completion and hence, long rig time and high well completion cost. Special tools have been developed to allow performing multiple tasks, such as setting plug, perforating, fracturing or cleaning, in one pipe trip to reduce rig cost, but at least one trip is required for each interval to be fractured and overall cost is still relatively high.
Another method that is commonly used to create multiple fractures in a single pumping stage is the use of diversion techniques, particularly the limited entry technique. The method of limited entry, such as that described in U.S. Pat. No. 4,867,241 (Strubhar) relies on high perforation entry friction to regulate fluid distribution into multiple perforated intervals. Some or all of the intervals are perforated with a limited number of holes, which causes an increase in pressure at the entrance of the perforations when the fracture treatment is pumped at high flow rate. The high entrance pressure forces fluid to enter multiple intervals, instead of entering only a single interval. Single stage treatment with diversion is less costly but uniform proppant placement is more difficult to achieve in multiple fractures and typically results in decreased well productivity. This is because the earth stress is seldom uniform even within a single rock formation. This causes fractures to be initiated in the lower stress intervals first. Once these fractures are initiated, they become the preferable flow path for the fracturing fluid being injected, leaving other perforated intervals unfractured. Even elevated treating pressure from the limited entry will not entirely mitigate this problem. Furthermore, as proppant enters the perforations, it erodes and enlarges the perforations, which causes the entry friction to decrease rapidly. As a result, the flow distribution among the multiple intervals is drastically altered when the proppant reaches the perforations. This causes a majority of the proppant to be placed only in a few dominant intervals, leaving other intervals unstimulated.
A method for producing multiple fractures from a single operation is described in U.S. Pat. No. 5,161,618 (Jones et al.). A plurality of packers are used to isolate the various intervals to be fractured, then a tool having a plurality of alternate paths or conduits and associated openings is used to supply fracturing fluid to different levels in the isolated interval or section. Each alternate path provided in the apparatus is associated with a specific set of holes or openings in the tool for providing fracturing fluid into the wellbore. Slurry is pumped through the conduits and fills the lower end of the tool prior to flowing into the wellbore, where it creates hydraulic pressure to fracture a first break-down zone. Slurry will continue to flow into this first zone until a bridge is formed or some other impediment to flow is created. At that point, the slurry will flow out of a second set of openings in the tool, which are positioned further up the wellbore to fracture a second break-down zone. However, providing slurry into a new fracture without first providing a clean fluid pad will typically cause the fracture to immediately screen out, thereby prohibiting further treatment of the fracture. Therefore, it would be advantageous to provide an apparatus that allows fracturing fluids to be provided to specific zones or intervals without the need for an alternate path for each zone and wherein the fluid delivered to each zone could be specifically controlled (i.e., providing a pad fluid prior to proppant slurry).
Yet another method for placing multiple fractures in horizontal wells is described in U.S. Pat. No. 6,070,666 (Montgomery). A tool having a packer and tubing for transporting a fracturing fluid and slump-inhibiting materials is used to produce multiple fractures in a horizontal wellbore. The tool is passed into the wellbore and positioned such that the packer may be inflated above a proposed fracture site, to effectively isolate the fracture zone (one end being sealed by the packer and the other end being the outer end of the horizontal well.) Fracturing fluid is then injected via the tubing to produce a fracture in the formation. Once the first fracture is formed, the tool must be withdrawn up the wellbore, where it is again put in place by inflating the packer and the fracturing process is repeated. This process may be used to produce any number of fractures; however, the tool must be moved for each new fracture site. It would be advantageous to provide a tool that could provide multiple fractures in a formation without requiring movement of the tool in the wellbore after each individual fracture was created.